1. Field of the Invention
This invention relates to the general area of remote site blood sample collection for medical diagnostic tests. More specifically, the invention relates to a improved filter paper for collecting a blood sample for higher precision analysis of hemoglobin or a hemoglobin fraction.
2. Description of Related Art
Filter paper has been used for more than 20 years as a means of gathering samples of blood, especially from newborns, for the purpose of identifying or measuring specific components in the blood in screening for congenital metabolic disorders, e.g., phenylketonuria (PKU). Although PKU screening has been one of the most common applications of blood collection on filter paper, many other applications have been in use, including the use of filter paper for collecting samples to test for blood components associated with thyroid dysfunction or in monitoring diabetic conditions. There are numerous papers in the scientific literature describing these testing methods. Although certain of these publications relate to characteristics of the filter paper itself, e.g., precision, lot-to-lot variations, and variability of results due to contamination of the filter paper used, most of the publications in the scientific literature focus on the methods employing a filter paper.
In 1984, a research paper was published on the use of filter paper for collecting blood samples for the purpose of measuring total glycohemoglobin. Eross, et al. (1984) Ann. Clin. Biochem. 21:477-483. Glycohemoglobin refers to the glycosylated state of hemoglobin resulting from glucose entering the red blood cell. Measurement of glycohemoglobin is extremely important in metabolic control of diabetics. Although blood glucose levels can fluctuate during the course of the day, relative to eating, medication, exercise, or other factors, glycohemoglobin levels are not so acutely affected. Thus, by measuring glycohemoglobin rather than glucose directly, a more accurate or more constant reading of blood glucose levels can be determined. Certain improvements to the total glycohemoglobin assay, which have been described in the scientific literature, have been achieved by modifying the filter paper used in the glycohemoglobin assay. Specifically, pretreatment of the filter paper with an enzyme, e.g., glucose oxidase, was used to reduce the glucose concentration in a blood sample being tested for total glycohemoglobin concentrations by Eross and by Little, et al. (1986) Clin. Chem. 32(5):869-871. The presence of glucose in the blood sample can cause an undesirable increase in glycohemoglobin concentrations compared to actual levels. A glycohemoglobin assay product using filter paper treated with glucose oxidase has been commercialized and was marketed under the trade name Self-Assure.TM., FlexSite Diagnostics, Palm City, Fla.
Although the use of a glucose oxidase-treated filter paper in measuring total glycohemoglobin has been described, such use has only been described in relation to affinity chromatography assays for total glycohemoglobin. Recently, methods have been introduced to measure a specific fraction of total glycohemoglobin, the so-called HbA.sub.1c fraction, Glucose glycates hemoglobin protein at a number of amino acid sites. About half of the glycation occurs at the N-terminal valine of the beta chain of hemoglobin. This glycation product is designated HbA.sub.1c. Antibody based methods for HbA.sub.1c have proved to be more specific than total glycohemoglobin methods. Thus, glucose present in the blood gives much larger errors in affinity chromatography methods for measuring total glycohemoglobin in dried blood spot than in an alternative method using an antibody assay, e.g., immunoprecipitation assays. Little, et al. (1996) Clin. Chem 42 (6): S193, showed that glucose present in a blood sample causes little interference in measuring the HbA.sub.1c fraction of glycohemoglobin using an antibody-based assay.
Two filter paper manufacturers, namely, Schleicher & Schuell (Keene, N.H., USA) and Whatman (Fairfield, N.J., USA) have devoted considerable commercial effort to refining certain of their filter paper products for application to dried blood spot methods, including glycohemoglobin testing. These refinements focus on consistency of the paper but not on treatment of the filter paper with various reagents. To date, it is not known to treat filter paper with a reagent other than glucose oxidase methods, according to Eross or Little, for use in a total glycohemoglobin assay.
In addition, it is not known that a treatment of the filter paper with a specific reagent can result in an improved assay for the HbA.sub.1c fraction of total glycohemoglobin by an antibody method. It is expected that when blood is applied to filter paper from a finger stick that coagulation will occur, thus binding the red blood cells into the clot matrix. It is further expected that when the blood is re-dissolved from the dried blood spot that coagulated blood will be more difficult to re-dissolve than un-coagulated blood, resulting in lower Hb concentrations in standard assays. Treatment with an anticoagulant of a filter paper used in an assay for measuring glycohemoglobin has not been heretofore described.